Chemical vapor deposition (CVD) methods are employed to form films of material on substrates such as wafers or other surfaces during the manufacture or processing of semiconductors. In CVD, a CVD precursor, also known as a CVD chemical compound, is decomposed thermally, chemically, photochemically or by plasma activation, to form a thin film having a desired composition. For instance, a vapor phase CVD precursor can be contacted with a substrate that is heated to a temperature higher than the decomposition temperature of the precursor, to form a metal or metal oxide film on the substrate.
Thin films that include ruthenium (Ru), ruthenium oxide (RuO2) or iron (Fe) have good electrical conductivity, high work function, are chemically and thermally stable, resistant to inter-layer chemical diffusion and are compatible with many dielectric substrate materials. Ru and RuO2 films, for instance, have been investigated as film electrode material for semiconductor devices such as DRAM (Dynamic Random Access Memory) devices.
Bis(pentahaptocyclopentadienyl)ruthenium(ruthenocene) and the symmetrical, diethyl-substituted ruthenocene(1,1′-diethylruthenocene) have been investigated as possible precursors for forming ruthenium-based thin films by CVD techniques. These compounds have been prepared by several synthetic routes.
One existing method for forming ruthenocene includes the reaction of RuCl3.XH2O with cyclopentadiene, in the presence of Zn, to produce ruthenocene, ZnCl2 and HCl, as shown in FIG. 1A. A similar approach, using ethyl-substituted cyclopentadiene, has been employed to produce 1,1′-diethylruthenocene, as shown in FIG. 1B. Generally, yields obtained by this method are about 70%.
As shown in FIG. 1C, unsubstituted ruthenocene also has been prepared by the reaction of cyclopentadiene, chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium(II) and sodium hydride (NaH) in benzene. Chloro(cyclopentadienyl)bis(triphenylphosphine)ruthenium(II) precursor has been synthesized by reacting ruthenium trichloride and triphenylphosphine in ethanol.
Another method that has been investigated for the synthesis of ruthenocene includes the transmetallation reactions of a bis(alkylcyclopentadienyl)iron compound with RuCl3.XH2O and results in the formation of low yield 1,1′-dialkylruthenocene, iron trichloride (FeCl3) and difficult to separate iron species.
As seen in FIGS. 1A and 1B, these synthetic approaches include a one step addition of both cyclopentadienyl rings and thus are suitable for preparing unsubstituted ruthenocene or the symmetrically substituted 1,1′-diethylruthenocene.
Monosubstituted ruthenocene, e.g., 1-ethylruthenocene, is formed as an impurity during the synthesis of 1,1′-diethylruthenocene. Another monosubstituted ruthenocene, tert-butyl(cyclopentadienyl)(cyclopentadienyl)ruthenium has been prepared by reacting a heated mixture of bis(cyclopentadienyl)ruthenium, aluminum chloride and polyphosphoric acid, with tert-butyl alcohol, followed by distillation.
Both ruthenocene and 1,1′-diethylruthenocene have relatively low vapor pressure (less than 10 Torr at 100° C.). At room temperature, ruthenocene is a solid and 1,1′-diethylruthenocene is a liquid.
Generally, more volatile CVD precursors are preferred, as are precursors that are liquid at room temperature, rather than solid. In addition, desired CVD precursors also are heat decomposable and capable of producing uniform films under suitable CVD conditions.
Therefore, a need exists for developing new ruthenocenes that are liquid at room temperature and have relatively high vapor pressure and for exploring their potential as CVD precursors for film depositions. A need also exists for developing other Group 8 (VIII) metallocene compounds that can be used as CVD precursors for forming osmium- or iron-based films.